The use of gas turbines (GTs) for electrical power generation can be very different in their working modus. GTs can be either used in order to produce a constant amount of electricity over a long period of time, as so-called “base loaders”, or they can be used in order to level the differences between the electricity production of rather constant sources (Nuclear, GT base loaders etc.) with addition of the variations due to the increasing amount of non-constant renewable energy and the non-constant electricity demand. The second type of GT is a so-called “cyclic/peaker”.
Within the lifetime of a GT it is possible that a “loader” becomes a “peaker”. This change in working conditions leads to differences in solicitations and distress modes (i.e. boundary conditions) for the components in the turbine and especially the ones subjected to extreme temperature conditions. In the case of “loaders” they will need a larger creep and oxidation resistance, and in the case of “peakers” those component will need a better cycling resistance.
Furthermore, for each component, and locally on the component, the boundary conditions are different. Some areas are more prone to fatigue and some other areas to creep, oxidation/corrosion, erosion, etc. All those properties are strongly depending on a coating that is usually used to adapt the component to the actual operational boundary conditions. In order to answer the variations in properties needed it is therefore of strong interest to be able to produce coatings with flexibly and individually tailored properties.
Regarding ductility, an environmental coating can provide improved oxidation and corrosion resistance; however it can cause problems with the mechanical property of the parts due to the low ductility of those coatings, especially at low temperatures. One approach in order to improve the ductility of the coating is to obtain a predominantly gamma′ structure that is modified with platinum group metal in order to avoid the formation of the beta nickel aluminide phase (brittle at low temperature), as it is explained in document US 2010/0330295 A1.
Another approach presented in document US 2012/0128525 A1, which also tries to optimize the composition of the bound coat, is trying to increase the gamma to gamma′ transition temperature with the addition of Tantalum (preferentially without Re). Tantalum stabilizes the formation of a three phase system (beta/gamma/gamma′) with a high gamma/gamma′ transition temperature (higher than the coating service temperature) allowing to reduce the local stresses.
Document US 2010/0330295 A1 mentioned above also claims to provide a ductile coating in which a plurality of compositional gradient layers can be used to form the ductile and oxidation/corrosion resistant coating. In document EP 2 354 454 A1 it is claimed that in order to reduce the coating costs, a turbine blade could be coated at different locations with coatings having different oxidation resistance. The locations of the part with lower working temperature could be coated with a less oxidation resistant coating, and the hot spots with a more oxidation resistant coating. The second coating can be either another coating or a modification of the first one.
A metallic-ceramic material with gradient of ceramic concentration and oxidation protection element has also been proposed in document WO 98/53940 A1. The concentration of ceramic is increasing toward the surface of the material, giving a higher temperature and oxidation resistance close to the surface.
Two documents mention the use of reservoir phase including a core-shell structure. Document U.S. Pat. No. 6,635,362 B2 claims the addition of an aluminum-rich phase, which comprises a core containing aluminum and a shell comprising an aluminum diffusion-retarding composition. However, no oxide shell is mentioned. In another document, US 2009/0202814 A1, a reservoir phase is claimed where a core shell structure is used. The shell can consist of a metal oxide. The core can also be granularly designed.
In general, the use of separate powder feeders for each separate powder which can be of either homogeneous composition or a flexible composite powder, thermally sprayed simultaneously where the ratio of each powder can be changed online by changing the feeding rate have never been mentioned in the prior art.
Document EP 1 712 657 A2 discloses a cold spray method for sequentially depositing a first powder material and a second powder material onto a substrate at a velocity sufficient to deposit said materials by plastically deforming the material without metallurgically transforming the powder. It is described that such cold spray technology is also applicable when the powdered materials may be fed to the nozzle using modified thermal spray feeders. The main gas is heated to 315° C. to 677° C., preferably 385° C. to 482° C. to keep it from rapidly cooling and freezing once it expands past the nozzle. The net effect is a desirable surface temperature on the substrate.
Document U.S. Pat. No. 5,705,231 discloses a method of producing a segmented abradable ceramic coating system including a base coat foundation layer, a graded interlayer and an abradable top layer, where the interlayer is applied by a spray gun and comprises a compositional blend of the base coat foundation layer and the abradable top layer. The three layer approach provides a means of tailoring the long-term thermal insulation benefit provided by the initial layers and the abradability benefit provided by the top layer.
The current state of the art in terms of overlay coatings or bond coat is to use coatings with a given composition within a strict range. Therefore, when compositional changes need to be performed in order to locally vary the properties of a coating in the X-Y plane (i.e. in a different area on the component), or in Z direction (i.e. with the depth of the coating), several powder types are used, with different compositions and they are then sprayed in a stepwise manner, leading to multilayer coatings or the use of two distinct coatings (with different compositions) at two distinct locations.
The usual multilayer concept is leading to misfit and irregularities between the different coatings layers. Furthermore, if one or more of the layers are detached the coating loses the corresponding property.
The use of a modular composite coating concept (as disclosed with the present invention) has never been reported. In order to get more freedom for relatively fast compositional changes the usual method used is to prepare powder blends. This means that the composition of each blend is determined once the blend is produced; in order to change the composition, a new blend has to be prepared.
Furthermore, in the state of the art, the powder needs to be changed in the powder feeders, leading to a loss of time, a loss of powder and a lack of flexibility. Powder blends have the disadvantage of de-mixing; they can usually only be used when the different powders have a similar density and particle size distribution; and their preparation is time consuming. This means that many combinations of different materials (metallic and ceramic) or powder with different size distributions (finer powder with a powder with larger particle sizes) can hardly be prepared as a blend. This is also one of the main reasons why multilayer coatings are used where each powder is sprayed separately.
When a coating is sprayed, usually 2 (in HVOF systems, as disclosed for example in document EP 1 816 229 A1 or EP 1 942 387 A1) up to 4 (in certain APS systems) powder feeders are used. However, the current state of the art is to feed the same powder with same composition in all the powder feeders. Therefore, each time the coating composition shall to be changed the powder feeders need to be emptied, cleaned and filled with the new powder.
It would therefore be of great advantage to use separated powder feeders for each powder and perform a modular spraying, where the compositional changes can be programmed in a spraying program for the full component. On this way, a coating system could be sprayed at once without changes of powder or interruptions in the spraying process.